1. Field of the Invention
The present invention relates to a wireless communication, and more particularly, to a method and apparatus of accessing a channel in a wireless communication system.
2. Related Art
With the advancement of information communication technologies, various wireless communication technologies have recently been developed. A wireless local area network (WLAN) is a technology whereby super high-speed internet access is possible in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.
Ever since the institute of electrical and electronics engineers (IEEE) 802, i.e., a standardization organization for WLAN technologies, was established in February 1980, many standardization works have been conducted. Initially, WLAN used a frequency of 2.4 GHz to support a data rate of 1 to 2 Mbps by using frequency hopping, spread spectrum, infrared ray communication, etc. Recently, the WLAN can support a data rate of up to 54 Mbps by using orthogonal frequency division multiplexing (OFDM). In addition, the IEEE 802.11 is developing or commercializing standards of various technologies such as quality of service (QoS) improvement, access point (AP) protocol compatibility, security enhancement, radio resource measurement, wireless access in vehicular environments, fast roaming, mesh networks, inter-working with external networks, wireless network management, etc.
The IEEE 802.11b standard supports a data rate of up to 11 Mbps (bits per second) by using a frequency band of 2.4 GHz. The IEEE 802.11a standard uses a frequency band of 5 GHz instead of the frequency band of 2.4 GHz and thus significantly reduces influence of interference. The IEEE 802.11a standard has improved the data rate to up to 54 Mbps by using the OFDM technology. The IEEE 802.11a standard uses a 20 MHz channel bandwidth within a bandwidth of 5 GHz. Although there are national differences, 13 channels can be used. The IEEE 802.11n standard provides increased network speed and reliability, extended coverage. The IEEE 802.11a standard uses a 20 MHz channel bandwidth or a 40 MHz channel bandwidth within a bandwidth of 5 GHz.
Basic access mechanism of an IEEE 802.11 is a carrier sense multiple access with collision avoidance (CSMA/CA) combined with binary exponential backoff. The CSMA/CA mechanism is also referred to as a distributed coordinate function (DCF) and basically employs a “listen before talk” access mechanism. A station (STA) listens a wireless medium before starting transmission. As a result of listening, if it is sensed that the wireless medium is not in use, the listening STA starts its transmission. Otherwise, if it is sensed that the wireless medium is in use, the STA does not start its transmission but enters a delay duration determined by the binary exponential backoff algorithm. The CSMA/CA channel access mechanism is not so efficient since throughput at the MAC layer provides only 50 to 60% of throughput at the physical layer.
IEEE 802.11 VHT (Very High Throughput) is one of WLAN systems which have been recently proposed to support throughput of higher than 1 Gbps. Two kinds of VHT system are independently progressed: one is IEEE 802.11ac below 6 GHz band and another is IEEE 802.11ac for 60 GHz band. In order to support throughput of higher than 1 Gbps, a feasibility test for 8×8 multiple-input multiple-output (MIMO) and 80 MHz channel bandwidth has been progressed. The VHT system is expected to use channel bandwidths broader than at least 80 MHz. The 80 MHz channel bandwidths can be configured by combining at least four adjacent 20 MHz channel. An AP simultaneously transmits data at different frequencies to multiple STAs to increase the overall throughput.
Meanwhile, IEEE 802.11y is an IEEE 802.11 amendment for operating the IEEE 802.11 between 3650 MHz and 3700 MHz. The IEEE 802.11y provides several key features such as contention based protocol (CBP), extended channel switch announcement (ECSA), and dependent station enablement (DSE). The extended channel switch announcement provides a mechanism for an access point to notify the stations connected to it of its intention to change channels or to change channel bandwidth. This mechanism will allow for the WLAN to continuously choose the channel that is the least noisy and the least likely to cause interference. This mechanism will also be used in the IEEE 802.11n, which will allow devices to switch between the IEEE 802.11y operation and the IEEE 802.11n operation in the 2.4 GHz and 5 GHz bands. The extended channel switch announcement also provides switching of a regulatory class. The regulatory class defines operating frequency band, transmit power limitation, etc.
The multi-band supporting protocol described above can be an essential feature of the VHT system. For the VHT system, the ECSA provided by the IEEE 802.11y needs to be enhanced. That is, for the VHT system which uses channel bandwidths broader than at least 40 MHz, a method of allocating and changing channel set is required. Also, a technique for efficiently operating a wideband VHT system is required.